Phycology Sectionin the German Society for Plant Sciences (DBG)

Sea Ice Alga Melosira arctica – winner or loser of climate change?

One of the most important microalgal species from the Arctic Ocean, Melosira arctica, has been nominated “Alga of the Year” by the German Phycology Section. Scientists will use Melosira as a model to understand consequences of climate change. “Currently no one can foresee whether Melosira will benefit or suffer from the melting of sea ice, and nobody knows why it is so productive under such hostile conditions,” says biologist Klaus Valentin from the Alfred Wegener Institute (AWI). He is a Member of the Phycological Section within the Germany Botanical Society, which selected Melosira as Alga of the year 2016.

“Our recent molecular studies indicate that the sea-ice diatom Melosira arctica is the most productive alga in ice-covered Arctic waters,” explains Klaus Valentin. For example, in 2013 Melosira was responsible for about 45% of Arctic primary production, indicating that the diatom produced a significant amount of biomass and oxygen at the expense of carbon dioxide.

Melosira measures about 30 micrometers (0.03 mm) in size and is surrounded by a shell made of silicates. Held together by a polysaccharide mucus, cells form long chains that can grow several metres long. They typically attach to the ice from underneath but Melosira can also grow within the ice and in meltwater ponds on top of the ice.

Melosira mats had already caught the attention of early Polar researchers accompanying Nansen on his expeditions in the 19th century. They collected samples, which today form part of the Hustedt Collection at the Alfred Wegener Institute.

It is yet to be established how Melosira survives the long and cold polar winters and can dominate the sea ice communities in spring and summer. Its life cycle is unknown this far, as are the specific factors governing its growth, such as light, nutrients, or salinity. This scientific gap regarding a key organism at the base of the Arctic food chain has motivated researchers to investigate Melosira in greater detail.

Systematic analyses possible for the first time

The project “Melosira arctica in a changing Arctic Ocean” is one of the first strategic initiatives of the AWI. The team has collected samples from locations throughout the Arctic Ocean and established unialgal cultures, enabling systematic analyses of the species for the first time.

The fundamental question is how Melosira will react to climate change, especially the decline of sea ice. Satellite measurements, which monitor changes in the extent of sea-ice, were initiated in 1979; since then, ice coverage has decreased significantly. Some climate models predict that the Arctic Ocean will be ice-free during summer as early as the second half of this century. “What will happen to Melosira which we currently find flourishing in perennial sea ice?” asks Valentin.

Physiological responses of ice algae

“Increasing temperatures and, along with the melting of sea-ice, increasing light intensities, should support the growth of microalgae. However, this does not necessarily apply to Melosira,” argues Valentin.

When algae photosynthesize, they produce oxygen; this gas can become trapped in the mucus of Melosira mats and increase their buoyancy. Perhaps Melosira can use this principle to survive in an ice-free Arctic Ocean? Researchers therefore plan to study the basic physiological requirements of Melosira: Which temperatures are optimal for growth and photosynthesis? What nutrients are necessary? What might be the consequence of, e.g., expected decreasing nitrate concentrations? What happens if the light climate changes? For example, decreases in ice coverage will enhance light intensities, which could be either beneficial or harmful for photosynthesis. Such important issues can best be addressed using controlled cultures in the laboratory rather than field studies.

Long-term adaptive capacity

An important question is whether Melosira has the potential to adapt to higher temperatures during rather fast global warming process. A long-term experiment shall help answer this question. Different isolates will be subjected to elevated temperatures for many (150) generations to elucidate whether some strains might be able to adapt to temperatures up to 8°C, with current growth conditions ranging from -2 to 0°C. If such temperature tolerant isolates can be found, Melosira could be able to survive expected changes in the Arctic Ocean.

Genetic variants

Research will also examine the genome of Melosira. Ultimately, the genetic make-up of a species carries the information necessary for survival and acclimation. It is therefore planned to compare the genome from Melosira strains from all over the Arctic, from the North Sea, the Baltic Sea and even from the River Weser.

Electron microscopic studies by Regine Jahn from the Botanical Garden of the Free Berlin University, currently deputy speaker of the Section Phycology, have confirmed morphological differences between isolates from Greenland and, e.g., Alaska and Canada. This is evidence for the existence of several genetic variants and thus the potential of Melosira to adapt to climate change.

Melosira also influences the deep sea

During years with a strong sea ice melt a mass sinking of algal mats to the sea floor can be observed which are then consumed by sea cucumbers, sea stars and bacteria; this has well documented during an Expedition of the research vessel ‘Polarstern’ in 2012. Such degradation processes consume oxygen and imply that Melosira also influences processes in the deep sea. The proportion of carbon fixed by the algae ‘exported’ by sinking can reach up to 85% of the total fixed carbon, considerably influencing the entire Arctic Carbon Cycle.

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